The present invention relates to a process for producing semiconductor devices. More specifically, the invention relates to a process for producing semiconductor devices while preventing thermal etching from taking place during the heat-treatment at high temperatures, enabling the yields of semiconductor devices to be increased.
In producing semiconductor devices, it is necessary to heat-treat the semiconductor wafers at high temperatures. For instance, when an impurity diffusion layer is to be formed on the main surface of a semiconductor wafer, it is accepted practice to dope the semiconductor wafer with impurities by ion implantation or the like, followed by heating at a high temperature, so that the impurities are diffused into the wafer. In a semiconductor device such as a complementary MOS (CMOS) IC containing CMOS transistors, in particular, it is essential to form a p-type or an n-type well diffusion layer in the semiconductor wafer to a depth of about 4 .mu.m. Therefore, the step of diffusion must be carried out in an nitrogen gas atmosphere heated at 1200.degree. C. for extended periods of time (see, for example, Japanese Patent Laid-Open No. 118366/1981).